The present invention relates to a photosensor for a semiconductor image sensor device which is used to convert incident radiation energy such as light radiation into electric signals.
Referring to the example using light as an electromagnetic radiation source, the prior art will be explained. FIG. 5 shows a conventional photosensor. A surface of a collector semiconductor substrate 5 of a bipolar transistor is covered with a silicon oxide film 6. When a light 9 is incident into the photosensor, electric charges are accumulated in a portion between a base 2 and a collector 5 and a base potential rises, then a base current flows from the base 2 to an emitter 1. The base current is amplified at a rate of static common-emitter current gain (hereinafter referred to as "hFE") and turns into a collector current IC, which flows from a collector terminal 51 to the emitter 1 via a collector electrode 8 and an emitter electrode 40.
FIG. 2 shows an equivalent circuit of a semiconductor image sensor device which uses a photosensor having the bipolar phototransistor. In FIG. 2, the photosensor shown in FIG. 5 is incorporated in the circuit as P1. When a switch S1 is turned on, the collector current IC is outputted to a capacitor C1 as a current. As an external output, the charges which accumulate in the capacitor C1 are outputted as an output voltage V01.
In order to read the intensity of incident light with this image sensor device, reproducibility and linearity are required of the relationship between an exposure and an output voltage. In FIG. 6, there is shown an emitter-base voltage--hFE characteristic of the conventional photosensor. The hFE changes depending on the base voltage, and is lower in the region in which the base voltage is lower. As the base voltage increases, the hFE becomes higher. Further, as FIG. 6 shows, this emitter-base voltage dependency of hFE is different in each device, and fluctuates even in the same device as time passes.
This phenomenon occurs mainly because carriers generated by light are bonded to surface states which exist in the surface of the base 2 below the silicon oxide film 6 in FIG. 5, and disappear. Therefore, the more surface states that exist, the more the hFE value declines in the region where the base voltage is lower. As most of these surface states are generated chiefly by dangling bonds of a silicon substrate, the dangling bond is restrained by adding hydrogen to it. However, the bonding strength of the hydrogen and the dangling bond is weak, and hydrogen easily diffuses in the silicon oxide film. Therefore, the restraint of the dangling bond with hydrogen is unstable. Moreover, the amount of surface states is different for different devices, and fluctuates by bias in the device, temperature and so on during the operation of a device. As a result, the emitter-base voltage dependency of hFE occurs and the dependency itself changes.
FIG. 7 shows a photoelectric transfer characteristic in the case of using such a conventional bipolar transistor as a photosensor. There occurs a phenomenon that the increased amount of the output voltage is smaller compared with the increased amount of the exposure in the region where the amount of exposure is smaller, and as the exposure increases, the increased amount of the output voltage increases. Further, there is a problem that as shown in FIG. 7 the photoelectric transfer characteristic is different depending on each device.
Therefore, the object of this invention is to obtain a photosensor in which the output voltage does not change by the amount of exposure and in which the increased amount of the output voltage against the increased amount of the exposure does not change depending on the amount of the exposure but is constant.